This invention relates to an electronic musical instrument having two tone signal generation circuits of different tone signal generation systems and being capable of forming a complex tone signal by combining tone signals generated by the two tone signal generation circuits.
This invention relates also to an electronic musical instrument having a tone signal generation circuit of a pitch synchronizing type and a tone signal generation circuit of a non-pitch-synchronizing type.
This invention relates also to a tone signal synthesis device for synthesizing a tone signal by employing a frequency modulation operation or an amplitude modulation operation and, more particularly, to such device capable of controlling a relatively numerous frequency components by a simple arithmetic operation.
Japanese Patent Application Laid-Open No. 58-102296 discloses an electronic musical instrument having two tone signal generation circuits of different tone signal generation systems, causing the two tone signal generation circuits to generate tone signals corresponding to the same nominal tone color selected by a performer or by other means at a common tone pitch and combining the two tone signals. In this electronic musical instrument, the first tone signal generation circuit has a memory storing tone waveforms of plural periods corresponding to various tone colors and generates a tone signal by reading out a tone waveform of plural periods corresponding to a selected tone color. The second tone signal generation circuit generates a tone signal corresponding to the selected tone color by executing a tone signal synthesis operation of a frequency modulation type. In a rise portion of a tone in which the tone waveform changes in a complex manner, a tone signal is generated by the first tone signal generation circuit and in a succeeding portion of the tone, a tone signal is generated by the second tone signal generation circuit and a single tone signal is synthesized by combining the two tone signals.
In the prior art electronic musical instrument in which generation of a tone signal in the rise portion is merely allotted to one tone signal generation circuit and generation of a tone signal in the succeeding portion is allotted to the other tone signal generation circuit, it is difficult to control tone synthesis and, besides, a performance effect obtained is a rather monotonous one.
Further, in the prior art electronic musical instrument, the first tone signal generation circuit and the second tone signal generation circuit perform the tone signal generation operation in accordance with a common sampling frequency and perform addition and synthesis of the two output tone signals at a common sampling clock timing. In this case, the first and second tone signal generation circuits respectively generate tone signals in accordance with a sampling frequency which is not synchronous with pitches of the tone signals. Particularly, in the second tone signal generation circuit which generates a tone signal by the tone synthesis operation of a frequency modulation type, it is difficult to adopt a circuit design in which a tone signal is generated in accordance with a sampling frequency which is synchronous with the pitch. This is because, in a tone signal generation circuit of a modulation operation type generally, it is difficult or cumbersome to adopt a circuit design in which a tone signal is generated in accordance with a sampling frequency which is synchronous with the pitch in executing arithmetic operation of each term of a frequency modulation operation formula on a time shared basis or generating tone signals in plural channels on a time shard basis, for there is limit in increasing the rate of the operation timing clock. For this reason, the first tone signal generation circuit also is obliged to have a circuit design in which a tone signal is generated in accordance with a sampling frequency which is not synchronous with the pitch.
However, in a case where a tone signal is generated in accordance with a sampling frequency which is not synchronous with the pitch, there arises, as is well known, the problem of occurrence of an aliasing noise which is not harmonic with the pitch of a generated tone.
Further, since in the prior art electronic musical instrument, output tone signals of the first and second tone signal generation circuits are used only for combination for synthesis of a single tone, it can hardly be said that the two tone signal generation circuits are fully utilized.
Known in the art is a technique for synthesizing a tone signal having a desired harmonic structure by a frequency modulation operation in an audio frequency range. In this prior art tone synthesis technique, a simple single-term frequency modulation operation is insufficient for synthesizing a tone of a satisfactory tone color having sufficient harmonic components and, for this purpose, a frequency modulation operation of a multiplex term or multiple terms must be executed. This requires a complex and bulky operation circuit and, in a system in which operation of each operation term is executed on a time shared basis, increase in the rate of a control clock with resulting increase in the manufacturing cost.
As a method for synthesizing a tone containing abundant harmonic components by a relatively simple operation, there has been proposed a method using a waveform having abundant frequency components as a modulating wave or a modulated wave. Since, however, a waveform which can be used for the operation is limited to one stored in a memory, there is limit to a tone color which can be synthesized.
For solving the above described problem, U.S. Pat. No. 4,766,795 discloses a technique according to which waveform data is stored in a logarithmic expression in a waveform table used for generation of a modulating or modulated wave and the waveform data in the logarithmic expression read from this table is multiplied with a coefficient whereby a complex waveform function is obtained by converting the waveform data in the logarithmic expression which is result of the multiplication to data in a linear expression. According to this technique, a modulating wave function or a modulated wave function can be easily converted to a complex function which is different from the function stored in the waveform table so that a tone signal containing abundant frequency components can be synthesized by a modulating operation with a relatively simple construction.
In the technique disclosed in the U.S. Patent, however, there is the problem that the technique can be applied to a case where waveform data is stored in a logarithmic expression in a waveform table but it cannot be applied to a case where waveform data is stored in a linear expression in the waveform table. Further, in a case where waveform data is stored in a logarithmic expression in a waveform table, a logarithm-linear conversion circuit is required in a posterior stage of the circuit.